.alpha.-Olefin hydroformylation processes catalyzed by triaryl phosphine rhodium complex catalysts were much studied. These studies were summarized in a recent monograph on, "New Syntheses with Carbon Monoxide," edited by J. Falbe, published by Springer-Verlag, Berlin, Heidelberg, New York, 1980. See the first chapter (pages 1 to 225) on, "Hydroformylation Oxo Synthesis Roelen Reaction," by B. Cornils. Another more specific summary has been published in Vol. 17 of Advances in Organometallic Chemistry, edited by F. G. Stone and R. West, published by Academic Press, New York, N.Y., 1979. See Chapter 1 (pages 1 to 60) on "Hydroformylation," by R. L. Pruett. The catalysis chemistry of rhodium hydroformylation was recently published by A. A. Oswald et al. in the Preprints of the Petroleum Chemistry Division of the American Chemical Society (Volume 27, Part 2, pages 292 to 309, covering the symposia of the Spring National Meeting, Mar. 28-Apr. 1, 1982.
The basic patent covering the commercial hydroformylation of propylene in the presence of triarylphosphine rhodium complexes is U.S. Pat. No. 3,527,809 by R. L. Pruett and J. A. Smith. Subsequently a number of improvement patents were issued.
Most importantly, U.S. Pat. No. 4,148,830 by R. L. Pruett and J. A. Smith claims the triphenyl phosphine rhodium complex catalyzed hydroformylation of .alpha.-olefins between 50.degree. and 145.degree. C. in a higher boiling aldehyde condensation product as a solvent and in the presence of excess triphenyl phosphine ligand. The significance of this patent is due to the occurrence of certain side reactions, i.e. an aldol addition of the aldehyde products followed by a Tischenko type reaction as it is shown below. ##STR2## The so called trimer and tetramer products of these reactions are much less volatile than the main aldehyde products. Thus upon the removal of the aldehyde products in the vapor phase, these aldehyde condensation products are enriched in the liquid reaction medium and automatically provide the claimed solvent.
Based on the Pruett and Smith inventions, E. A. V. Brewester and R. L. Pruett developed a low pressure continuous product flash-off process using the triphenylphosphine rhodium complex based catalyst system in the aldehyde condensation products as solvent medium. This process is disclosed in U.S. Pat. No. 4,247,486. In this process, the olefin, CO and H.sub.2 reactants plus fresh feed are recycled. Of course, an enrichment of the liquid reaction mixture in the earlier discussed higher boiling aldehyde condensation products necessarily occurs in the PFO operation disclosed by Brewester and Pruett in U.S. Pat. No. 4,247,486.
Based on the above patents, a successful, low pressure, low temperature continuous product flash-off process was commercially developed for the hydroformylation of propylene in the presence of the above triphenylphosphine rhodium complex based system (See Chemical Enginnering, page 110, Dec. 5, 1977). This process has eliminated the rhodium losses common to processes employing a chemical recovery of the catalyst. However, the known PFO catalyst system is not completely stable. Consequently, there are many patents aimed at restoring the activity of the catalyst system.
A major effort was also made on improving the catalyst activity maintenance. U.S. Pat. No. 4,227,627 by D. R. Bryant and E. Billig disclosed three critical parameters for activity maintenance in PFO hydroformylations. According to this patent, the activity maintenance, i.e. catalyst stability factor, is directly related to the triphenylphosphine to rhodium ratio and indirectly related to the CO partial pressure and temperature. These parameters were correlated to maximize the stability of the trimer solvent based catalyst system.
In the above patent, Bryant and Billig state that in a commercial operation a realistic loss of catalyst activity is 0.5% per day. They point out that using their invention this loss can be reduced to below 0.3% per day. However, as it is shown by their examples, Billig and Bryant obtained a catalyst activity loss of 2% or higher per day at 120.degree. C. or above, due to the strong adverse effect of temperature on stability. Consequently, the Billig and Bryant process is limited to temperatures from about 90.degree. to about 130.degree. C.
The selectivity of the known continuous hydroformylations was expected to be adversely affected by high reaction temperatures. G. Montrasi et al. published that the amount of by-product heavy ends, specifically the "trimers," is strongly proportional to the reaction temperature. See Volume 6, pages 737 to 742 of the La Chimica e L' Industria journal in Milan, 1980.
In further improvement patents, similar low temperature processes are claimed in excess triphenyl phosphine as a solvent. U.S. Pat. No. 4,108,905 by G. Wilkinson discloses the hydroformylation of propylene and other gaseous olefins with triphenyl phosphine rhodium complexes in liquid triphenyl phosphine medium, in the absence of liquid olefin using H.sub.2 /CO reactant ratios of about 2 to 0.07. Wilkinson notes that at 150.degree. C. and above his catalyst decomposes. Wilkinson's examples included experiments with continuous product removal by distillation and analysis by gas chromatography. However, he did not observe any aldolization products. The duration of his experiments is not disclosed. A short running time would explain the apparent absence of higher boiling by products. Canadian Pat. No. 992,101 by J. H. McCracken and R. C. Williamson also discloses similar hydroformylation in triphenyl phosphine using varying H.sub.2 /CO ratios but being limited to temperatures between 60.degree. to 115.degree. C. and batch operation.
U.S. Pat. No. 4,229,990 by H. Tummes, B. Cornils and H. Noeske describes the stabilization of triphenylphosphine rhodium complex catalysts in triphenylphosphine solvent by C.sub.1 to C.sub.5 paraffinic hydrocarbons employed as a component of the olefinic feed in a continuous PFO operation. According to this patent, there was a rapid decrease in catalyst activity in the absence of paraffins which acted as an extra component of the stripping gas in removing the aldehyde products. The Tummes et al. patent did not disclose the enrichment of the liquid catalyst system in aldehyde condensation products.
The decrease of the activity of the triphenylphosphine rhodium complex system was at least in part attributed to the formation of n-alkyl diphenyl phosphine ligands. See U.S. Pat. No. 4,151,209 by J. L. Paul, W. L. Pieper and L. W. Wade and U.S. Pat. No. 4,260,828 by D. G. Morrell and P. D. Sherman, Jr. and a journal article in volume 62, pages 389-394 of La Chimica e L' Industria, Milan, 1980, by G. Gregorio, G. Montrasi, M. Tampieri, P. Cavalieri d' Oro, G. Pagani and A. Andreetta. The Paul et al. patent additionally emphasized the adverse effect of even higher boiling organophosphorus by-products.
The paper by Montrasi et al. pointed out that for example in propylene hydroformylation, the formation of propyldiphenylphosphine is directly related to the propylene reactant concentration. Alternatively, inactive phosphido complexes are formed at low propylene concentration. Furthermore, triphenylphosphine is oxidized to triphenylphosphine oxide by aldehydes, particularly by 2-ethylhexenal, according to Montrasi et al. In general, the scientific literature is highly pessimistic regarding a substantially complete stabilization of the homogeneous liquid catalyst systems based on triarylphosphine rhodium complexes.
It is apparent from the prior art that 1-n-olefins are much more reactive in phosphine rhodium complex catalyzed hydroformylation than internal or vinylically substituted olefins. It was disclosed in U.S. Pat. No. 4,287,370 by N. Harris, A. J. Dennis and T. F. Shevels that a mixture of isomeric butenes can be used as a feed in a continuous PFO process using a trimer solvent for the selective conversion of the 1-butene component to n-valeraldehyde.
Attempts were also made to utilize the triphenylphosphine rhodium complex catalyst system in the conversion of C.sub.4 -C.sub.6 internal linear olefins to the corresponding n-aldehydes via combined isomerization-hydroformylation. According to U.S. Pat. No. 4,200,592 by R. R. Hignett and P. J. Davidson only about a 0.16 ratio of n- to i-valeraldehyde products is obtained in such a process starting with 2-butene. However, Hignett et al. disclose that the n/i product ratio can be somewhat increased in the presence of added cocatalysts based on complexes of other transition metals.
In contrast to the known prior art, it was surprisingly found in the present invention that superior high temperature hydroformylation selectivity and excellent operational stability are achieved using homogeneous liquid triarylphosphine rhodium complex catalyst systems when the excess triaryl phosphine ligand is a major solvent component and a sufficient concentration of dissolved CO reactant is maintained to avoid CO starvation.
It was shown in the batch hydroformylation studies of the triphenylphosphine rhodium complex based catalyst systems that superior hydroformylation selectivities are obtained when the concentration of the excess triphenyl phosphine ligand is between 1 and 2.9 molar and when the H.sub.2 /CO ratio is maintained between 2.5 and 10. In this preferred concentration range of excess ligand, the hydroformylation catalyst system is much more stable and provides a higher n- to i-ratio of aldehyde products than at the lower concentrations disclosed in the Pruett and Smith patents. Furthermore, the present catalyst system also provides a higher ratio of n/i products than the triphenyl phosphine only process of the Wilkinson patent.
In addition, it was found that the catalyst system is much less sensitive to higher partial pressures of carbon monoxide when the phosphine concentration is high. In effect, both the apparent rate of the hydroformylation and its selectivity to total aldehydes increases with increasing CO partial pressures. In contrast, the Pruett et al. and Bryant et al. patents disclose a process limited to a top CO partial pressure of 55 psia.
Surprisingly, the selectivity of the present catalyst system with regard to the n-versus i-ratio of the aldehyde products was found to be insensitive to the triaryl phosphine to rhodium ratio. Under a certain set of reaction conditions, the selectivity is essentially determined by the triaryl phosphine concentration alone. The rhodium concentration can be independently increased to achieve the desired reaction rate without affecting the n/i ratio of products.
In the continuous high temperature hydroformylation studies of various triphenylphosphine rhodium complex based catalyst systems it was surprisingly found that trimer solvent formation by Pruett et al, and Montrasi et al., can be substantially eliminated. In the continuous process of the present invention the formation of the higher molecular weight by products is suppressed. The key intermediate of the secondary higher molecular weight by-products was found to be the unsaturated aldol aldehyde rather than the hydroxy aldol aldehyde. As shown below, during the present hydroformylation process, the unsaturated aldol aldehyde by-product undergoes hydrogenation and subsequent aldolization, oxidation and esterification reactions. Consequently, the resulting monoalcohols and monoesters rather than the trimers are enriched in the liquid reaction mixture during product distillation and as such become the high boiling solvent components ##STR3##
In addition, the liquid reaction media of the continuous process of the present invention contain significant amounts of dissolved olefin in the liquid phase. Thus the reaction occurs in the liquid phase. This is in contrast to the disclosures of the Wilkinson patent which specifies hydroformylation of a gaseous olefin in the absence of liquid olefin reactant. The present reaction media also contain increased amounts of dissolved CO as a result of increased CO partial pressures, in excess of 25 psia. The latter is in contrast to the minimum CO partial pressures preferred in the Pruett et al. and Bryant et al. patents.
As a result, the high temperature continuous hydroformylation process of the present invention shows a unique combination of selectivity and maintenance of catalyst activity. In contrast to the Tummes et al. and Paul et al. patents the maintenance of catalyst activity in these systems is not due to either the presence of alkanes in the feed or the absence of high boiling organophosphorus by-products in the mixture. In the present selective process for n-aldehyde production, the stability is believed to be mainly attributable to the maximization of the rhodium species present as the tris-triarylphosphine rhodium carbonyl hydride complex via the special reaction medium and the above discussed operational parameters.
As a result of the understanding of the various factors controlling the complex equilibria among the various complexes of the triarylphosphine rhodium based catalyst complexes, the present invention also provides an improved combined isomerization hydroformylation process for the selective conversion of internal olefins to n-aldehydes at minimum CO partial pressure. In contrast to the Hignett et al. patent, the present process does not require the presence of transition metals other than rhodium.
Finally, it is surprisingly found that the present high temperature process is also applicable for the selective hydroformylation of internal linear olefins to i-aldehydes, at maximum CO partial pressures.